This invention relates to triode type field emitter devices for high frequency amplification and switching systems, and more particularly to methods and apparatus for operation of such devices with collector initiated, gate modulated field emission.
Field emission devices for signal switching and amplification that utilize structures with one or more field emitters are well known in the art. These devices may be fabricated on a substrate with a configuration that is arranged laterally or vertically with respect to a planar surface of the substrate. Such devices have been designed with well known configurations such as electron sources, diodes and triodes. Electron sources generally comprise a field emitter with a proximate extraction gate control electrode that initiates and controls current flow from the tip of the field emitter toward and through the extraction gate according to the well known Fowler-Nordheim relationship between field emission and electric field applied to the emitter by the extraction gate. The extraction gate has at least one aperture through it to allow some proportion of the emitter field emission to pass through the gate.
Diodes generally comprise the field emitter with a proximate extraction control electrode or collector that initiates and controls current flow from the tip of the field emitter to the collector according to the Fowler-Nordheim relationship. Triodes generally comprise the field emitter with a combination of the collector and an intermediately positioned extraction gate control electrode. The electric field applied to the emitter by the extraction gate serves to initiate and control the current flow to the collector according to the Fowler-Nordheim relationship. The collector in turn collects the emission from the emitter that passes through the extraction gate.
Using known field emitter devices, field emission from the emitter requires that the extraction gate must always be maintained at some positive potential relative to the emitter, because in such devices the emission is initiated and sustained only because of the field applied to the emitter tip according to the Fowler-Nordheim relationship. In contrast, the potential applied to the collector has only a small effect upon emission and collector current.
Known field emitter devices have several serious disadvantages that limit their use for high frequency signal amplifiers and flat panel fluorescent displays. One of these disadvantages is the high gate-to-emitter capacitance that is caused by the close proximity of the gate apertures to the emitter tips. The close proximity, typically in the range of 0.5 to 1.0 .mu.m, is necessary to achieve low device turn-on potential, typically in the range of 60 to 100 V. This high input capacitance limits the high frequency performance of these devices due to capacitive reactance.
Another disadvantage of known field emitter devices is the high gate leakage current that occurs at moderate collector potentials. The gate leakage current increases proportionately as collector potential decreases because the number of electrons that have their paths redirected from the gate to the collector diminishes.
Still another disadvantage is high dynamic output resistance. This occurs because the field emission initiated by the extraction gate limits the number of electrons that can reach the collector, so that saturation of collector current develops with even moderate collector potentials. The high resulting output resistance makes efficient high frequency output coupling difficult when even small amounts of capacitive reactance are present in the output circuit.